Geomembrane liners are made of large sheets of flexible plastic or resilient material. The sheets are assembled, being one or more, to define a barrier which contains liquids in an impoundment. Such a liquid impoundment is normally built on the face of the earth after shaping a shallow pond. The pond may be filled with high quality drinking water. It may be used to store hazardous waste materials for subsequent processing and disposal. In general terms, the impoundment is normally intended to be secured gainst leakage and, to this end, the geomembrane liner is built to define the pond. Leaks must be detected and repaired to avoid significant leakage, potential ground water contamination, or loss of valuable product. The present disclosure is particularly directed to an impoundment made of two or more geomembrane liners and set forth a system of inspection so that leaks can be detected and located in any of the layers.
This disclosure thus relates to a geomembrane liner installation having an upper liner spaced above a lower liner. The two generally extend to the same marginal edge and are joined at the surrounding edge. The surrounding earth is normally shaped to define a shallow pond. The pond thus comprises the storage area above both liners. The risk of leakage through the lower liner makes detection all the more difficult because it is so totally obscured by the upper liner placed above it.
The system set forth by this disclosure has advantages over other methods of inspection. For example, monitoring wells around and perhaps extending beneath the pond can be used. Measurements so obtained through monitoring wells are generally indirect. Moreover, they are delayed in response time; by the time that a leak is detected through this approach, substantial leakage has already occurred and significant damage may well occur as a result of the untended leak. Monitoring wells can be rather shallow or can extend much deeper into strate therebelow which serve as aquifers. Detection of leaked constituents in the aquifer, even with detection that measures parts per billion is acceptable, but again it is significantly delayed and is therefore less desirable than the approach of the present disclosure. One alternate approach involves U.S. Pat. No. 4,543,525 directed to a directional probe system reading on a galvanometer. This system has different sensitivities along different probe angular positions. Therefore, a leak can be missed entirely by wrong probe orientation.
The present apparatus and method disclosed herein takes advantage of the fact that an electric current can be conducted by a liquid between the two liners. To this end, the liquid placed between the two liners is selected so that it is conductive at least to a specified level. A suitable power supply is connected between a first electrode in contact with the earth outside of the lower liner and a second electrode in contact with the liquid between the two liners. This arrangement represents an electrical circuit in which current may flow through the lower geomembrane which, by virtue of its material properties, is a high resistance interface between the first electrode and conducting earth and the second electrode and conducting liquid. As long as the lower liner is in perfect condition, and no leak has occurred, current flow will be of very low magnitude. In the event that a leak occurs in the lower liner, the conductive liquid passing through the perforation (be it large or small) will allow a measurable current flow. This current passes through the leak and produces an adjacent magnetic field which is detected in the manner and by the means described hereinbelow. An important factor to note is that leakage can be detected from inception of the perforation in the lower liner, and the position of such leakage can be quickly localized. This is particularly important in view of the fact that the lower liner is obscured. Moreover, the pond secured by the impoundment above the two liners can encompass many acres and thereby be quite large. This large size can nevertheless be safely and easily surveyed to detect and locate leaks by application of the present invention.